Conventional residential circuit breakers include a molded case having multiple case pieces (e.g., halves or thirds) forming one or more internal cavities that are configured to accept the various internal circuit breaker components. Some circuit breakers, such as all mechanical single-pole circuit breakers, may include a mechanism pole with a first and second molded case piece housing the internal tripping mechanism components therein. In two-pole electronic circuit breakers, such as ground fault circuit interrupters (GFCIs) type and combination arc fault circuit interrupters (CAFCI) type residential two-pole circuit breakers, an electronic pole may be sandwiched between two mechanism poles. In each case, the mechanism poles may include a moveable contact arm with an attached moveable electrical contact, a stationary electrical contact mounted to the molded case, and a tripping mechanism adapted to separate the stationary and moveable electrical contacts when tripped.
Conventionally, in one mode of operation, the tripping mechanism allows for manual tripping via throwing a handle of the circuit breaker, but also includes a bi-metal and magnet mechanism that allows for: 1) tripping of the circuit breaker in a thermal mode by motion of the bimetal of a bimetal and magnet mechanism engaging an armature due to internal resistive heating due to a persistent overcurrent situation, and 2) for instantaneous tripping due to high current through the bimetal and magnet mechanism of a magnitude of 5× or more than the rated handle current of the circuit breaker.
During an instantaneous tripping event, the armature is magnetically attracted to the magnet by magnetic forces generated in the magnet due to the conductance of the high current through the current path including the bimetal. Instantaneous level, as used herein, is the current level at which the circuit breaker will trip due to the abnormally high current (e.g., 5× current) passing through the circuit breaker. As the armature rotates, the latch bite of a latch between the armature and a cradle of the tripping mechanism is decreased. Eventually, when the latch bite gets very small, the cradle is disengaged from the armature and the tripping mechanism will trip. Upon tripping, the moveable and stationary electrical contacts will separate creating an electrical open in the protected branch circuit.
In existing circuit breaker designs, the magnet is typically welded to the bimetal and the magnet. Because the magnet is welded to the bimetal, the air gap that exists between the magnet and armature is dependent on the bimetal position in the molded case. However, the position of the bimetal may vary due to the welding operations involved in the assembly process. Each time the bimetal is heated to create a welded joint, the bimetal is damaged in the localized areas and may warp and move. Additionally, the bimetal may be welded to connect the load terminal and current braid. Welding processes are controlled by weld parameters for the welding machine, but typically have a relatively large range to work within and thus variations of the bi-metal configuration may occur.
Because of the possibility of warping, once the welding has been completed, the circuit breaker is thermally calibrated to ensure desired thermal tripping is actually achieved, depending on circuit breaker type. Calibration may be accomplished by adjusting a calibration screw of a thermal calibration mechanism coupled to a portion of the conductive path, such as by adjusting a position of a strap coupled to the bimetal.
However, calibration of the thermal tripping may inadvertently change the instantaneous level of the circuit breaker. Accordingly, circuit breakers that can achieve more reliable instantaneous trip level are desired.